<html>

<head>
    <script type="text/javascript">
        function validate_email(field, alerttxt) {
            with(field) {
                apos = value.indexOf("@")
                dotpos = value.lastIndexOf(".")
                if (apos < 1 || dotpos - apos < 2) {
                    alert(alerttxt);
                    return false
                } else {
                    return true
                }
            }
        }

        function validate_form(thisform) {
            with(thisform) {
                if (validate_email(email, "Not a valid e-mail address!") == false) {
                    email.focus();
                    return false
                }
            }
        }
    </script>
</head>

<body>
    <form action="https://www.baidu.htm" onsubmit="return validate_form(this);" method="post">
        Email: <input type="text" name="email" size="30">
        <input type="submit" value="Submit">
    </form>
</body>

</html>